The present invention generally relates to an apparatus for detecting failures of a longitudinal acceleration sensor for detecting longitudinal acceleration of a motor vehicle and more particularly, to an apparatus which is capable of detecting not only a small-gain failure that output of the longitudinal acceleration sensor is smaller than actual acceleration of the motor vehicle but a large-gain failure that output of the longitudinal acceleration sensor is larger than actual acceleration of the motor vehicle and is properly used for an antiskid control system.
Conventionally, in order to improve control accuracy of an antiskid-control system, it has been proposed to utilize signals from a longitudinal acceleration sensor for detecting longitudinal acceleration of a motor vehicle. The antiskid control system performs control on the premise that the longitudinal acceleration sensor is normal. Therefore, in case failures of the longitudinal acceleration sensor happen such that output value of the longitudinal acceleration sensor is smaller or larger than actual acceleration or deceleration of the motor vehicle, performance of the antiskid control system in which the signals from the longitudinal acceleration sensor are used as control data deteriorates greatly.
For example, in an antiskid control system in which the signals from the longitudinal acceleration sensor are used for performing estimated calculation of vehicle body speed, brake fluid pressure is reduced, increased or held on the basis of wheel speed which is calculated from vehicle body speed estimated from signals outputted by the longitudinal acceleration sensor and output of wheel speed sensors. Therefore, if vehicle body speed calculated from output of the longitudinal acceleration sensor is different from actual vehicle body speed, such problems arise that braking distance increases due to insufficient brake fluid pressure or wheels are locked due to excessive brake fluid pressure.
In order to solve these problems, an apparatus for detecting failures of the longitudinal acceleration sensor has been proposed so far. For example, Japanese Patent Laid-Open Publication No. 2-284068 (1990) proposes an apparatus which, if it is found that output of a longitudinal acceleration sensor has a value indicative of acceleration or deceleration of a motor vehicle when the motor vehicle is running at a constant speed, judges the longitudinal acceleration sensor to be defective so as to perform fail-safe processing.
However, this known apparatus is capable of detecting failures of the longitudinal acceleration sensor in case output is generated from the longitudinal acceleration sensor in spite of a state that the motor vehicle is neither accelerated not decelerated or in case output of the longitudinal acceleration sensor is larger than actual acceleration or deceleration of the motor vehicle (large-gain failure). On the other hand, the known apparatus is not capable of detecting failures of the longitudinal acceleration sensor when output is not generated from the longitudinal acceleration sensor in spite of a state that the motor vehicle is being accelerated or decelerated or when output of the longitudinal acceleration sensor is smaller than actual acceleration or deceleration of the motor vehicle (small-gain failure). Therefore, in an antiskid control system incorporating the known apparatus, in case small-gain failure happens, it is judged that road surface has low coefficient .mu. of friction even if road surface actually has high coefficient .mu. of friction, so that control for road surface having low coefficient .mu. of friction is performed, thereby resulting in increase of stopping distance, etc.
Meanwhile, Japanese Patent Laid-Open Publication No. 4-110267 (1992) proposes an apparatus for detecting failures of a longitudinal acceleration sensor and a transverse acceleration sensor for detecting transverse acceleration of a motor vehicle. When it is found that output of the longitudinal acceleration sensor has a value indicative of acceleration or deceleration of the motor vehicle when the motor vehicle is running at a constant speed, the apparatus judges the longitudinal acceleration sensor to be defective. Meanwhile, if output level of the transverse acceleration sensor is not less than a predetermined level when there is no difference among wheel speeds of respective wheels, namely, the motor vehicle is running straight-forwardly, the apparatus judges the transverse acceleration sensor to be defective. Furthermore, in case output level of the transverse acceleration sensor is less than the predetermined level when difference between wheel speeds of the left and right wheels is not less than a predetermined value, namely, the motor vehicle is turning, the apparatus judges the transverse acceleration sensor to be defective.
However, in this prior art apparatus, the transverse acceleration sensor is capable of detecting both large-gain failure and small-gain failure but the longitudinal acceleration sensor is not capable of detecting small-gain failure. Therefore, in this prior art apparatus, the same problem as that of the above mentioned known apparatus arises.